Diesel Engine Fundamentals Part 4

DIESEL ENGINE SPEED, FUEL C ONTROLS, AND PROTECTION Understanding how diesel engines are controlled and the types of protective instrumentation available is important for a complete understanding of the operation of a diesel engine. EO 1.7 DESCRIBE how the mechanical-hydraulic governor on a diesel engine controls engine speed. EO 1.8 LIST five protective alarms usually found on mid-sized and larger diesel engines. Engine Control The control of a diesel engine is accomplished through several components: the camshaft, the fuel injector, and the governor. The camshaft provides the timing needed to properly inject the fuel, the fuel injector provides the component that meters and injects the fuel, and the governor regulates the amount of fuel that the injector is to inject. Together, these three major components ensure that the engine runs at the desired speed. Fuel Injectors Each cylinder has a fuel injector designed to meter and inject fuel into the cylinder at the proper instant. To accomplish this function, the injectors are actuated by the engine's camshaft. The camshaft provides the timing and pumping action used by the injector to inject the fuel. The injectors meter the amount of fuel injected into the cylinder on each stroke. The amount of fuel to be injected by each injector is set by a mechanical linkage called the fuel rack. The fuel rack position is controlled by the engine's governor. The governor determines the amount of fuel required to maintain the desired engine speed and adjusts the amount to be injected by adjusting the position of the fuel rack. Each injector operates in the following manner. As illustrated in Figure 26, fuel under pressure enters the injector through the injector's filter cap and filter element. From the filter element the fuel travels down into the supply chamber (that area between the plunger bushing and the spill deflector). The plunger operates up and down in the bushing, the bore of which is open to the fuel supply in the supply chamber by two funnel-shaped ports in the plunger bushing. Figure 26 Fuel Injector Cutaway The motion of the injector rocker arm (not shown) is transmitted to the plunger by the injector follower which bears against the follower spring. As the plunger moves downward under pressure of the injector rocker arm, a portion of the fuel trapped under the plunger is displaced into the supply chamber through the lower port until the port is closed off by the lower end of the plunger. The fuel trapped below the plunger is then forced up through the central bore of the plunger and back out the upper port until the upper port is closed off by the downward motion of the plunger. With the upper and lower ports both closed off, the remaining fuel under the plunger is subjected to an increase in pressure by the downward motion of the plunger. When sufficient pressure has built up, the injector valve is lifted off its seat and the fuel is forced through small orifices in the spray tip and atomized into the combustion chamber. A check valve, mounted in the spray tip, prevents air in the combustion chamber from flowing back into the fuel injector. The plunger is then returned back to its original position by the injector follower spring. On the return upward movement of the plunger, the high pressure cylinder within the bushing is again filled with fresh fuel oil through the ports. The constant circulation of fresh, cool fuel through the injector renews the fuel supply in the chamber and helps cool the injector. The fuel flow also effectively removes all traces of air that might otherwise accumulate in the system. The fuel injector outlet opening, through which the excess fuel returns to the fuel return manifold and then back to the fuel tank, is adjacent to the inlet opening and contains a filter element exactly the same as the one on the fuel inlet side. In addition to the reciprocating motion of the plunger, the plunger can be rotated during operation around its axis by the gear which meshes with the fuel rack. For metering the fuel, an upper helix and a lower helix are machined in the lower part of the plunger. The relation of the helices to the two ports in the injector bushing changes with the rotation of the plunger. Changing the position of the helices, by rotating the plunger, retards or advances the closing of the ports and the beginning and ending of the injection period. At the same time, it increases or decreases the amount of fuel injected into the cylinder. Figure 27 illustrates the various plunger positions from NO LOAD to FULL LOAD. With the control rack pulled all the way (no injection), the upper port is not closed by the helix until after the lower port is uncovered. Consequently, with the rack in this position, all of the fuel is forced back into the supply chamber and no injection of fuel takes place. With the control rack pushed all the way in (full injection), the upper port is closed shortly after the lower port has been covered, thus producing a maximum effective stroke and maximum fuel injection. From this no-injection position to the full-injection position (full rack movement), the contour of the upper helix advances the closing of the ports and the beginning of injection. Figure 27 Fuel Injector Plunger Governor Diesel engine speed is controlled solely by the amount of fuel injected into the engine by the injectors. Because a diesel engine is not self-speed-limiting, it requires not only a means of changing engine speed (throttle control) but also a means of maintaining the desired speed. The governor provides the engine with the feedback mechanism to change speed as needed and to maintain a speed once reached. A governor is essentially a speed-sensitive device, designed to maintain a constant engine speed regardless of load variation. Since all governors used on diesel engines control engine speed through the regulation of the quantity of fuel delivered to the cylinders, these governors may be classified as speed-regulating governors. As with the engines themselves there are many types and variations of governors. In this module, only the common mechanical-hydraulic type governor will be reviewed. The major function of the governor is determined by the application of the engine. In an engine that is required to come up and run at only a single speed regardless of load, the governor is called a constant-speed type governor. If the engine is manually controlled, or controlled by an outside device with engine speed being controlled over a range, the governor is called a variablespeed type governor. If the engine governor is designed to keep the engine speed above a minimum and below a maximum, then the governor is a speed-limiting type. The last category of governor is the load limiting type. This type of governor limits fuel to ensure that the engine is not loaded above a specified limit. Note that many governors act to perform several of these functions simultaneously. Operation of a Governor The following is an explanation of the operation of a constant speed, hydraulically compensated governor using the Woodward brand governor as an example. The principles involved are common in any mechanical and hydraulic governor. The Woodward speed governor operates the diesel engine fuel racks to ensure a constant engine speed is maintained at any load. The governor is a mechanical-hydraulic type governor and receives its supply of oil from the engine lubricating system. This means that a loss of lube oil pressure will cut off the supply of oil to the governor and cause the governor to shut down the engine. This provides the engine with a built-in shutdown device to protect the engine in the event of loss of lubricating oil pressure. Simplified Operation of the Governor The governor controls the fuel rack position through a combined action of the hydraulic piston and a set of mechanical flyweights, which are driven by the engine blower shaft. Figure 28 provides an illustration of a functional diagram of a mechanical-hydraulic governor. The position of the flyweights is determined by the speed of the engine. As the engine speeds up or down, the weights move in or out. The movement of the flyweights, due to a change in engine speed, moves a small piston (pilot valve) in the governor's hydraulic system. This motion adjusts flow of hydraulic fluid to a large hydraulic piston (servo-motor piston). The large hydraulic piston is linked to the fuel rack and its motion resets the fuel rack for increased/decreased fuel. Figure 28 Simplified Mechanical-Hydraulic Governor Detailed Operation of the Governor With the engine operating, oil from the engine lubrication system is supplied to the governor pump gears, as illustrated in Figure 29. The pump gears raise the oil pressure to a value determined by the spring relief valve. The oil pressure is maintained in the annular space between the undercut portion of the pilot valve plunger and the bore in the pilot valve bushing. For any given speed setting, the spring speeder exerts a force that is opposed by the centrifugal force of the revolving flyweights. When the two forces are equal, the control land on the pilot valve plunger covers the lower ports in the pilot valve bushing. Figure 29 Cutaway of a Woodward Governor Under these conditions, equal oil pressures are maintained on both sides of the buffer piston and tension on the two buffer springs is equal. Also, the oil pressure is equal on both sides of the receiving compensating land of the pilot valve plunger due to oil passing through the compensating needle valve. Thus, the hydraulic system is in balance, and the engine speed remains constant. When the engine load increases, the engine starts to slow down in speed. The reduction in engine speed will be sensed by the governor flyweights. The flyweights are forced inward (by the spring), thus lowering the pilot valve plunger (again, due to the downward spring force). Oil under pressure will be admitted under the servo-motor piston (topside of the buffer piston) causing it to rise. This upward motion of the servo-motor piston will be transmitted through the terminal lever to the fuel racks, thus increasing the amount of fuel injected into the engine. The oil that forces the servo-motor piston upward also forces the buffer piston upward because the oil pressure on each side of the piston is unequal. This upward motion of the piston compresses the upper buffer spring and relieves the pressure on the lower buffer spring. The oil cavities above and below the buffer piston are common to the receiving compensating land on the pilot valve plunger. Because the higher pressure is below the compensating land, the pilot valve plunger is forced upward, recentering the flyweights and causing the control land of the pilot valve to close off the regulating port. Thus, the upward movement of the servo-motor piston stops when it has moved far enough to make the necessary fuel correction. Oil passing through the compensating needle valve slowly equalizes the pressures above and below the buffer piston, thus allowing the buffer piston to return to the center position, which in turn equalizes the pressure above and below the receiving compensating land. The pilot valve plunger then moves to its central position and the engine speed returns to its original setting because there is no longer any excessive outward force on the flyweights. The action of the flyweights and the hydraulic feedback mechanism produces stable engine operation by permitting the governor to move instantaneously in response to the load change and to make the necessary fuel adjustment to maintain the initial engine speed. Starting Circuits Diesel engines have as many different types of starting circuits as there are types, sizes, and manufacturers of diesel engines. Commonly, they can be started by air motors, electric motors, hydraulic motors, and manually. The start circuit can be a simple manual start pushbutton, or a complex auto-start circuit. But in almost all cases the following events must occur for the starting engine to start. 1. The start signal is sent to the starting motor. The air, electric, or hydraulic motor, will engage the engine's flywheel. 2. The starting motor will crank the engine. The starting motor will spin the engine at a high enough rpm to allow the engine's compression to ignite the fuel and start the engine running. 3. The engine will then accelerate to idle speed. When the starter motor is overdriven by the running motor it will disengage the flywheel. Because a diesel engine relies on compression heat to ignite the fuel, a cold engine can rob enough heat from the gasses that the compressed air falls below the ignition temperature of the fuel. To help overcome this condition, some engines (usually small to medium sized engines) have glowplugs. Glowplugs are located in the cylinder head of the combustion chamber and use electricity to heat up the electrode at the top of the glowplug. The heat added by the glowplug is sufficient to help ignite the fuel in the cold engine. Once the engine is running, the glowplugs are turned off and the heat of combustion is sufficient to heat the block and keep the engine running. Larger engines usually heat the block and/or have powerful starting motors that are able to spin the engine long enough to allow the compression heat to fire the engine. Some large engines use air start manifolds that inject compressed air into the cylinders which rotates the engine during the start sequence. Engine Protection A diesel engine is designed with protection systems to alert the operators of abnormal conditions and to prevent the engine from destroying itself. Overspeed device - Because a diesel is not self-speed-limiting, a failure in the governor, injection system, or sudden loss of load could cause the diesel to overspeed. An overspeed condition is extremely dangerous because engine failure is usually catastrophic and can possibly cause the engine to fly apart. An overspeed device, usually some type of mechanical flyweight, will act to cut off fuel to the engine and alarm at a certain preset rpm. This is usually accomplished by isolating the governor from its oil supply, causing it to travel to the no-fuel position, or it can override the governor and directly trip the fuel rack to the no-fuel position. Water jacket - Water-cooled engines can overheat if the cooling water system fails to remove waste heat. Removal of the waste heat prevents the engine from seizing due to excessive expansion of the components under a high temperature condition. The cooling water jacket is commonly where the sensor for the cooling water system is located. The water jacket temperature sensors provide early warning of abnormal engine temperature, usually an alarm function only. The setpoint is set such that if the condition is corrected in a timely manner, significant engine damage will be avoided. But continued engine operation at the alarm temperature or higher temperatures will lead to engine damage. Exhaust temperatures - In a diesel engine, exhaust temperatures are very important and can provide a vast amount of information regarding the operation of the engine. High exhaust temperature can indicate an overloading of the engine or possible poor performance due to inadequate scavenging (the cooling effect) in the engine. Extended operation with high exhaust temperatures can result in damage to the exhaust valves, piston, and cylinders. The exhaust temperature usually provides only an alarm function. Low lube oil pressure - Low oil pressure or loss of oil pressure can destroy an engine in short order. Therefore, most medium to larger engines will stop upon low or loss of oil pressure. Loss of oil pressure can result in the engine seizing due to lack of lubrication. Engines with mechanical-hydraulic governors will also stop due to the lack of oil to the governor. The oil pressure sensor usually stops the engine. The oil pressure sensors on larger engines usually have two low pressure setpoints. One setpoint provides early warning of abnormal oil pressure, an alarm function only. The second setpoint can be set to shutdown the engine before permanent damage is done. High crankcase pressure - High crankcase pressure is usually caused by excessive blow-by (gas pressure in the cylinder blowing by the piston rings and into the crankcase). The high pressure condition indicates the engine is in poor condition. The high crankcase pressure is usually used only as an alarm function. Summary The important information in this chapter is summarized below. Diesel Engine Speed, Fuel Controls, and Protection Summary A mechanical-hydraulic governor controls engine speed by balancing engine speed (mechanical flyweights) against hydraulic pressure. As the engine speeds up or slows down, the weights move the hydraulic plunger in or out. This in turn actuates a hydraulic valve which controls the hydraulic pressure to the buffer piston. The buffer piston is connected to the fuel rack. Therefore, any motion of the buffer piston will control fuel to the cylinder by adjusting the position of the fuel rack, which regulates the amount of fuel in the injectors. Most mid-sized to large diesel engines have (as a minimum) the following protective alarms and trips. Engine overspeed alarm/trip High water jacket temperature alarm High exhaust temperature alarm Low lube oil pressure (alarm and/or trip) High crankcase pressure alarm